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Using the Low Frequency Array (LOFAR), European astronomers have investigated a galaxy cluster designated CIZA J2242.8+5301, dubbed the Sausage cluster. The observations conducted at very low radio frequencies provide more insights into the properties of radio relics in this cluster. The new findings are presented in a research paper May 29 on the arXiv preprint server.

Galaxy clusters consist of up to thousands of galaxies bound together by gravity. They are the largest known gravitationally-bound structures in the universe, and therefore serve as excellent laboratories for studying galaxy evolution and cosmology. Observations show that galaxy clusters generally form as a result of mergers and grow by accreting sub-clusters.

CIZA J2242.8+5301 is a well-studied merging galaxy cluster at a redshift of 0.192. It contains prominent double radio relics (diffuse, elongated radio sources of synchrotron origin) and other diffuse radio sources. CIZA J2242.8+5301 was nicknamed the Sausage cluster due to the distinctive morphology of its northern relic.

Recently, astronomers led by Giulia Lusetti of the University of Hamburg, Germany, have made use of LOFAR's Low Band Antenna (LBA) to perform the lowest radio frequency (45 MHz) study of the Sausage cluster. Their main goal was to investigate the cluster's radio relics.

"Observations at frequencies below 100 MHz are essential for investigating the physics of radio relics as they provide unique access to the low-energy population of cosmic-ray electrons," the researchers explained.

The observations found that the Sausage cluster has a complex system of diffuse, relic-like sources, extending beyond the well-known northern and southern relics. Many of these sources have a head-tail morphology and show interactions with the extended emission.

The northern relic exhibits a characteristic arc-like or sausage-like morphology with a projected linear size of about 7.2 by 2.5 million light years. The southern relic showcases a more irregular shape for a total extent of about 4.9 by 1.7 million light years.

Based on the spectral index maps between 45 MHz and 144 MHz, the astronomers found a clear spectral gradient in both relics as the spectral index steepens from the outer edge towards the cluster center. The integrated spectral index of the northern relic was measured to be −1.09, while the southern relic has a steeper integrated index of approximately −1.34. These values indicate Mach numbers at a level of 4.8 and 2.6, for the northern and southern relics, respectively.

The researchers also employed a model to simulate the surface brightness profile of the northern relic to gain insights into cluster dynamics. They found that this relic is best represented by a scenario with minimal projection effects, magnetic field variation, and shock deformation.

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